The present invention relates to a press device having an extended nip intended for pressing a running paper or paperboard web. More specifically, the invention relates to a shoe press having a support that supports a press shoe adjacent to a counter roll or another backing member in such a way that the press shoe and the backing member form an extended nip therebetween, and having a hydraulic or pneumatic arrangement for urging the press shoe toward the backing member in order to apply pressure on the web passing through the nip.
The press device according to the invention particularly advantageously can be utilized for wet-pressing of moist paper or paperboard webs, but also for calendering or other finishing of dried paper or paperboard. The press device according to the invention can also be utilized for fiber webs other than paper and paperboard.
Shoe press devices having an extended nip have been employed for many years when manufacturing different paper and paperboard grades, primarily for wet-pressing in order to increase the dryness of the web, but also for calendering and other finishing in order to improve surface properties or other physical properties of the web.
As a result of their longer nips, shoe press devices provide a number of advantages in comparison to conventional roll presses, such as a higher dryness at the same nip pressure, or the ability to press at a lower at nip pressure while maintaining the same dryness, which is more gentle to the sheet. U.S. Pat. No. 4,917,768 discloses a shoe press device in which the press shoe is supported via tubular sleeves rigidly affixed to and spaced apart on a support in a direction across the machine direction. The sleeves are received in cylindrical recesses in the press shoe in order to allow the press shoe to move toward or away from a counter roll such that the nip pressure can be varied. The disclosed shoe press comprises hydraulic jacks upstream and downstream of the sleeves for forcing the press shoe against the counter roll and for pivoting the shoe about an axis that extends in the cross-machine direction in order to vary the nip pressure in the machine direction. The sleeves fit somewhat loosely in the recesses in the shoe, and a resilient seal encircles each sleeve for sealing the interface between the sleeve and the recess. Accordingly, the press shoe of the shoe press disclosed in U.S. Pat. No. 4,917,768 is capable of pivoting relative to the support for varying the nip pressure in the machine direction.
However, it has been found that such shoe presses can be associated with certain problems. One such problem originates from the thermal expansion of the press shoe, which is a result of the heat generated by friction against the belt that runs over the press shoe and carries the paper or paperboard web through the press and by the hot hydraulic fluid that for different reasons is circulated through the shoe. The thermal expansion of the press shoe results in an elongation of the shoe in the cross-machine direction, which creates bending tensions in the support and hydraulic arrangement of the press shoe, which of course is undesirable.
In EP 0 933 471, corresponding to U.S. Pat. No. 6,083,352, the disclosure of which is incorporated herein by reference, a shoe press is disclosed that reduces the problems originating from the thermal expansion of the press shoe, since it has the ability to tolerate relatively large elongations of the press shoe across the machine direction, and also other deformations of the press shoe. The disclosed shoe press comprises a press shoe extending in the cross-machine direction along the entire width of a web running through the press, and a plurality of articulated hydraulic loading cylinders supported by a support and spaced apart along the shoe. The loading cylinders define working chambers that are pressurizable by hydraulic fluid, so as to enable the cylinders to urge the press shoe away from the support and toward a counter roll or other backing member for applying pressure to the web being carried through the nip defined between the shoe and the backing member. Each loading cylinder comprises a piston member disposed within a cylinder member. Either the piston or the cylinder comprises a two-part member having a first member fixed relative the press shoe and a second member fixed relative to the support, while the remaining piston or cylinder comprises a coupler.
In a preferred embodiment of the shoe press according to EP 0 933 471, the two-part member consists of first and second cylinders, while the coupler comprises a piston that is slidably received in the two cylinders. In an alternative preferred embodiment the two-part member instead comprises first and second pistons, while the coupler comprises a cylinder that surrounds both pistons.
The coupler of the shoe press disclosed in EP 0 933 471 sealingly engages at least one of the members, so that the first member is urged away from the second member in a loading direction when pressurizing the working chamber to cause the press shoe to be urged towards the backing member. In order to enable the loading cylinders to accommodate elongation of the press shoe across the machine direction, each coupler engages the respective first and second members at seals that enable the coupler to pivot relative to the first and second members about axes parallel to the machine direction. Accordingly, the press shoe is free to expand thermally in the cross-machine direction without causing bending of any piston and/or cylinder members of the loading cylinders.
Since the press shoe of the shoe press disclosed in EP 0 933 471 also can move or be tilted in the machine direction in relation to the carrier, the shoe press comprises a stopping member that restricts the movement of the shoe forwards in the machine direction.
Accordingly, the loading cylinders of the above-described types of shoe press devices have piston and cylinder members that can be tilted in relation to each other and/or in relation to the press shoe in order to accommodate movements of the press shoe in relation to the carrier across the machine direction, and also in the machine direction. Even if such tilting is necessary, in reality it is undesired since it can result in elastic seals of the loading cylinders being subjected to large stresses, or in very large forces being directed against the guiding or stopping means that in accordance with the present techniques are necessary for ensuring that the press force exercised by the press shoe is directed in the intended direction towards the counter member when lengthening the loading cylinders. Therefore, it would be advantageous to provide a press device having loading cylinders that allow skewness between the relatively slidable cylinder members included in the loading cylinder when necessary, but that otherwise counteract such skewness.
Accordingly, the present invention seeks to provide a press device having an extended nip for pressing a running paper or paperboard web, in which device the loading cylinders when necessary allow but otherwise counteract skewness between the relatively slidable cylinder members, and which ensures that the lengthening force of the loading cylinders in all circumstances are directed in the intended direction, in order to thereby minimize the stresses acting on the elastic seals of the loading cylinders and the forces that during the pressing may come to be directed towards guiding or stopping means for the press shoe of the press device.
In accordance with the invention, a press device comprises a press shoe extending across the machine direction and arranged for forming an extended nip in cooperation with a counter member for passage of the web during the pressing. The press device further comprises a carrier supporting the press shoe such that the press shoe is displaceable toward and away from the counter member by a plurality of loading cylinders spaced apart along the press shoe for enabling application of pressure onto the web during the pressing. At least a few of the loading cylinders comprise a first cylinder member having a first end attached to or integrated in the press shoe, and a second cylinder member having a second end attached to or integrated in the carrier. The first and the second cylinder member are slidably coupled to each other by a generally tubular coupling member. The coupling member has a first portion that slidably engages the first cylinder member and is displaceable over a first length of stroke L1 relative to the first cylinder member, and a second portion that slidably engages the second cylinder member and is displaceable over a second length of stroke L2 relative to the second cylinder member. The first portion of the coupling member is closer to the carrier than is the second portion of the coupling member.
In another aspect of the invention, a first seal is disposed between the coupling member and the first cylinder member for sealing the connection therebetween, and a second seal is disposed between the coupling member and the second cylinder member for sealing the connection therebetween, such that a sealed working chamber is formed in the loading cylinder. Lateral displacement of the press shoe relative to the carrier exerts a moment on the coupling member causing the coupling member to pivot about a central axis of the second cylinder member, and the cylinder members and coupling member are arranged such that said pivoting of the coupling member exposes surfaces of the coupling member to fluid pressure in the working chamber and said surfaces are located such that the fluid pressure acting on said surfaces generates a moment on the coupling member in a direction opposite to that exerted by the displacement of the press shoe. Accordingly, the coupling member tends to be restored toward its original undisplaced position.